Bodily infection is a major concern for patients having implanted structures such as total knee or total hip replacements. In that any object implanted into the human body is a foreign object, regardless of the duration of time it has existed within the body, infection originating from even remote areas of the body can cause complications with the implanted object. For example, if the patient were to become infected from a rusted nail, the infection would likely spread to the tissue surrounding a joint implant.
Several different procedures have been developed in the past to prevent the formation and spread of infection in an implant area. One such procedure has been to administer large doses of antibiotics to the patient in hopes of eradicating the infection all together. Although this procedure is very simple and straightforward, it unfortunately rarely cures the patient of the infection by itself.
A more effective solution has been to remove the implant and to replace it with some form of spacer containing an antibiotic agent. For example, in the case of total knee replacement patients, physicians will remove the femoral and tibial prostheses and the polyethylene "meniscus" positioned therebetween and replace them with a puck-shaped spacer made of bone cement impregnated with an antibiotic. Typically, the spacer is formed during the operation on a side table in the operating room so that the surgeon can first inspect the joint to determine what size spacer will be needed. After the appropriate size has been ascertained, the surgeon or an assistant simply mixes the constituent elements of the bone cement, typically a polymer and a monomer that will form polymethyimethacrylate, and adds an appropriate amount of antibiotic, typically in powder form. Once the bone cement begins to harden, it becomes malleable and may be shaped by hand to fit within the implant area of the patient where the primary prosthesis had once been. When the spacer has adequately hardened, it is then inserted into the implant area and the incision closed in the conventional manner.
Positioned in the implant area, the antibiotic is slowly leached out from the bone cement and into the surrounding tissue to prevent the formation and spread of the infection.
While the infection is being fought, typically over a period of 4 to 6 weeks, the bone cement spacer maintains the amount of joint spacing that was present in the implant area prior to removal of the primary prosthesis. If some form of spacer were not placed in the implant area, this joint spacing would be substantially reduced due to the contraction of the muscles and tendons of the patient during the recuperation period.
Antibiotic-impregnated bone cement has also been utilized to prevent the formation and spread of infection in total hip replacement patients. In this scenario, the acetabular cup and the hip stem of the primary hip prosthesis are first removed. Next, the surgeon mixes the polymer, monomer, and the antibiotic agent and forms small beads of bone cement. These beads of bone cement are then positioned in a spaced relationship along a length of stainless steel suture wire. Once the bone cement has adequately hardened, the wire together with the beads is inserted into the femoral canal to destroy any infection present within the femur. After the infection has been overcome, the surgeon withdraws the beads of bone cement by simply pulling the suture wire out of the femoral canal, carefully counting the number of beads removed to ensure that no beads remain in the femur.
Unlike the knee spacer, the bead and suture wire device does not maintain the joint spacing of the implant area since the device does not occupy any of the acetabular cavity that the head and the acetabular cup of the primary hip prosthesis previously occupied. Since the joint will be immobilized and the patient often bedridden for a substantial period of time, contraction of the unused muscles and tendons can cause substantial joint spacing shrinkage. Although this shrinkage is not in itself harmful to the patient, it will shorten the patient's limb requiring the patient to either walk with a limp or use some form of orthotic device to compensate for the shortened limb. While it is sometimes possible for the physician to stretch the muscles and tendons of the patient to restore some of this lost spacing, it is unlikely that the patient will regain the full amount of joint spacing he or she had before the primary hip prosthesis was removed.
Accordingly, it can be seen that it would be desirable to provide a method and apparatus for fighting infection in an implant area of a patient which also maintains adequate joint spacing.